Dominant phytoplankton groups as the major source of polyunsaturated fatty acids for hilsa Tenualosa ilisha in the Meghna estuary Bangladesh

The tropical estuarine ecosystem is fascinating for studying the dynamics of water quality and phytoplankton diversity due to its frequently changing hydrological conditions. Most importantly, phytoplankton is the main supplier of ω3 polyunsaturated fatty acids (PUFA) in the coastal food web for fish as they could not synthesize PUFA. This study evaluated seasonal variations of water quality parameters in the Meghna River estuary (MRE), explored how phytoplankton diversity changes according to hydro-chemical parameters, and identified the major phytoplankton groups as the main source of PUFA for hilsa fish. Ten water quality indicators including temperature, dissolved oxygen, pH, salinity, dissolved inorganic nitrogen (DIN = nitrate, nitrite, ammonia) and phosphorus, dissolved silica and chlorophyll-a were evaluated. In addition, phytoplankton diversity was assessed in the water and hilsa fish gut. Principal component analysis (PCA) was used to analyze the spatio-temporal changes in the water quality conditions, and the driving factors in the MRE. Four main components were extracted and explained 75.4% variability of water quality parameters. The most relevant driving factors were dissolved oxygen, salinity, temperature, and DIN (nitrate, nitrite and ammonia). These variabilities in physicochemical parameters and dissolved inorganic nutrients caused seasonal variations in two major groups of phytoplankton. Peak abundance of Chlorophyta (green algae) occurred in water in nutrient-rich environments (nitrogen and phosphorus) during the wet (36%) season, while Bacillariophyta (diatoms) were dominant during the dry (32%) season that depleted dissolved silica. Thus, the decrease of green algae and the increase of diatoms in the dry season indicated the potential link to seasonal changes of hydro-chemical parameters. The green algae (53.7%) were the dominant phytoplankton group in the hilsa gut content followed by diatoms (22.6%) and both are contributing as the major source of PUFAs for hilsa fish according to the electivity index as they contain the highest amounts of PUFAs (60 and 28% respectively)

Aquaculture and Fisheries in the Sundarbans and Adjacent Areas in Bangladesh: Resources, Productivity, Challenges and Opportunities

Estuarine aquatic systems and braided rivers in and adjacent to the Sundarbans and the vast area of the Ganges tidal floodplain next to the core forest area in Bangladesh holds rice aquatic faunal diversity and provides plenty of opportunities to grow fish, shrimps and crabs. Currently these systems provide direct employment opportunity for 1.2 million people and indirect or seasonal livelihood for more than 10 million people across the southwest coast. Hilsa is the largest fishery in this region and shrimp brings the highest cash and export earnings. Proper implementation of fisheries regulation is critical to ensure conservation of the rich fish diversity of this region as well as to continue to support livelihood of millions of people living on fisheries. While aquaculture is the major contributor to national fish production, agricultural GDP and export earning, it requires planned advancement from the current state to continue to grow in harmony with environment. Integration with rice and other crops, and with mangrove wherever possible can bring long-term sustainability of these systems. Change in the river flow due to siltation and reduced upstream flow, climate change, sea level rise, outbreak of disease in fish and crustaceans are major challenges for future growth and sustainability of both aquaculture and fisheries in this region. Collaborative and multi-disciplinary research should be undertaken to address these challenges. In addition, there should be mechanism to bring research outputs into use and make impact on sustainability